Small-scale and decentralised wastewater treatment plant optimisation through real-time control

Abstract

There is increasing (and sometimes contradictory) demand on operators of small and decentralised wastewater treatment plants (WWTPs) to improve biological nutrient removal and energy efficiency. However such WWTPs can be subject to unique design and operational challenges that can impact performance including; (i) lack of permanent operators and local expertise, (ii) relatively high energy cost, (iii) sludge handling, (v) non-consistent influent hydraulic or organic loads and (vi) inflexible operating regimes. These challenges, when combined with increasingly stringent legislation in the European Union (and by extension Ireland), are key drivers for the development of innovative operation and control solutions for WWTPs. A sequencing batch reactor (SBR) is a technology commonly applied to smaller WWTPs. SBRs are batch operated systems, a feature which can be advantageous in coping with non-consistent influent hydraulic or organic loads, however, they are inflexible in terms of operating regimes, typically operated using fixed time treatment cycles. There are a number of other technologies that utilise batch treatment arrangements and these provide significant opportunities for the implementation of novel control and operation systems to optimise environmental and energy efficiencies. Real time control (RTC) procedures (automated monitoring and control) of WWTPs have significant potential to optimise plant performance and energy efficiency. Online sensors for key parameters, including ammonium-nitrogen (NH4-N), can provide data on the operation of the WWTP while also allowing the application of RTC procedures; for example terminating a treatment cycle when NH4-N concentrations are lowered to an acceptable level and thus saving energy. However, NH4-N sensors can require excessive maintenance, are unreliable unless frequently maintained and are often not affordable. However, the use of cost efficient and reliable surrogate sensors, such as oxidation reduction potential (ORP) and pH, can be adopted for use in RTC procedures. RTC procedures have been successfully adapted within SBRs. Examples include intelligent software sensor based systems which utilise numerical inferential models including; neural networks (NN), multiple linear regression (MLR), and various fuzzy techniques. However, as RTC procedures require expensive control equipment and technical knowledge to operate they have not been widely applied to smaller WWTPs. Much of the published research is limited to laboratory based facilities which are not subject to the variation typically experienced by small and decentralised WWTPs. Thus new control methods that can operate in real-life conditions, and on “low-tech” control units are required to enhance the performance and energy efficiency of small and decentralised wastewater WWTPs. In this study two RTC procedures were developed to terminate treatment cycles when certain NH4-N conditions were achieved using pH and ORP sensors. These procedures comprised two approaches namely; (i) low resource procedures - three methods were developed for use with low cost programmable logic controllers (PLCs) incapable of running complex mathematical control algorithms; and (ii) advanced procedures – these sought to develop numerous softsensors using complex algorithms for use with compatible PLC’s. Four batch operated test units were used to gather trend data for this study, two SBR units and two pumped flow biofilm reactor (PFBR) units (a passively aerated biofilm based system), namely; SBR 1 – a pilot scale unit, SBR 2 – a field scale unit, PFBR 1 – a field scale unit and PFBR 2 – a laboratory scale unit. The application of each developed procedure was optimised for site specific characteristics as well as site specific goals. This was achieved by analysing various control procedures using a ranked assessment criteria developed in this study, This was designed to allow a control approach to be selected that would enable the specific goals at that site be prioritised (for example an operative might seek to prioritise energy savings over NH4-N removal if they were not subject to a stringent discharge limit). The developed procedures can be used to optimise WWTP performance and can be adapted to existing WWTPs with little on-going maintenance, thus making them a viable solution for aiding control systems and environmental engineers operating small and decentralised WWTPs.